Re-re-re-thinking the rise of mammals and death of the dinosaurs

Placental mammals may have waited for the asteroid to hit after all.

Grandma? An artist's conception of the appearance of the first placental mammal.

Carl Buell

The end of the era of dinosaurs and rise of the mammals has held a fascination for me since I was young (and I suspect I'm not alone). But it's a tale that has been retold many times now.

In the earliest versions, I recall ideas about clever, fast-moving mammals outcompeting the slow, lumbering dinosaurs. With time, however, that story changed. The dinosaurs became quicker and actually survived, albeit as birds (papers now refer to the loss of "non-avian dinosaurs"). The mammals became less clever and more lucky, in that it took a freak hit from an asteroid to trigger the mass extinction. As DNA data came in, the amount of luck involved seemed more and more significant. Data indicated some of the lineages of modern placental mammals had been around for millions of years before the dinosaurs died, but didn't really do much until after the extinction event.

A new analysis, published in Science now pushes back against the molecular data. A large team of authors tracked thousands of individual traits in more than 125 species (40 of them known only from fossils) to build the biggest reconstruction of the history of mammals ever attempted. In doing so, they find the first placental mammal probably didn't exist until after the non-avian dinosaurs were gone, the study even provides some hints of what it might have looked like.

Reconstructing the history of mammals is challenging for a lot of reasons. To begin, their history in the fossil record is incomplete. For example, we know the lemurs are related to lorises, but diverged before their home island, Madagascar, was separated from nearby continents. However, there simply isn't any hint of lemurs in the fossil record; they were what's termed a "ghost lineage" for tens of millions of years.

Other complications come from the fact that the first big radiation of placental mammals looks a bit like an explosion. It took millions of years for ecosystems to recover after the death of the dinosaurs but, once they did, new species and major groups of mammals appeared in quick succession, making the precise order in which new groups arose difficult to determine.

All of which made DNA work rather challenging. The timing of some splits can be estimated using the degree of difference between the DNA of existing species, but there still needs to be something to anchor these "molecular clocks." In most cases, that involves finding a split in the fossil record that we can assign a precise date to. That's difficult when the fossils tell a somewhat confused tale.

The new tree is informed by molecular data—the authors now have more than 25 mammalian genomes to work with—but is mostly focused on morphological traits (the shapes of bones and organs). Although individual traits can get confused as some are lost and others evolve independently, a large catalog like the one used by the authors can provide a clear picture of how species relate to each other. Reaching back through fossils, it can also provide an indication of when different species branched off.

Enlarge/ A mammal that dates to the Cretaceous and has a mix of marsupial and placental traits.

AMNH/S. Goldberg and M. Novacek

The authors conclude some mammalian lineages are quite old. Monotremes like the platypus and echidna, for example, may go back roughly 200 million years to the Triassic. Marsupials split off in the Jurassic. There are a few extinct lineages that are more closely related to us than marsupials, but the new tree clearly excludes them from the placentals (and none of them survived the mass extinction). The ancestor of the placentals seemed to evolve within a couple of million years of the mass extinction, and the diversification really got going about five million years after.

Because the tree is based on shared traits, the authors were able to reconstruct what the ancestral species probably looked like. These include some remarkable details, including the paths traveled by specific nerves (which can be preserved on bone surfaces) to major features like the presence of large regions of the brain devoted to smell, and the fact that males carried their testes inside their abdomen. More generally, the animal was likely to be a small insectivore, looking a bit like a hybrid of a North American opossum and a rat.

The unexpected aspect of this late arrival on the scene is the first placentals appeared after Pangea was well on its way to breaking up, meaning mammals would have had to cross some significant bodies of water. It also means some of them moved around quite a bit. Fossil evidence, for example, indicates the Afrotheria, a group that includes animals like elephants and aardvarks, probably got its start in North or South America, although later extinctions removed most members of the group from those continents.

Although this study is impressively comprehensive, it shouldn't be viewed as the last word. There's still some work to do to get this tree in better accordance with the DNA data, which itself is changing with the development of more sophisticated methods for generating a molecular clock. Meanwhile, because of the importance of fossil data to the tree, a significant find could potentially alter its shape. The story of mammals may end up being retold again.

42 Reader Comments

In "The Ancestor's Tale", Richard Dawkins presented an age of 105 million years for the last common ancestor of placental mammals. If I recall correctly, his information was based on "molecular clock" data. Is it correct to say that his clock was running slow, and this data is basically recalibrating it?

Anyways, this is really cool. I'm sure that the creation "scientists" will try to come up with some way to fit their narrow and biased viewpoint, but this is still a great day for unlocking the past of us mammals.

It strikes me that this is certain. The story will be retold, over and over, in ever-finer detail, likely with occasional large revisions in accepted wisdom. Really deep history like this is an intricate mystery, and the theory of the case will constantly be changing as technologies and academic disciplines improve. There will always be more detail to fill in; we'll never have the whole story.

Hell, we don't have the full story about things that are happening now, much less millions of years ago.

Well how about insects and animals except for the plants have eyes, noses, and mouths. Their internal organs have intestines and a stomach to digest food with? These similarities are more convincing to me than just the placental alone.

Convincing for what? That we share a common ancestor with those organisms? We certainly do. However that common ancestor would be an older one than the common ancestor of placental mammals. Think of it like this:

Placental Mammals - animals that have bilateral symmetry, whose mouth forms second in early development, who have a notochord during development, who have backbones, who have jaws, who have lobe-limbs, who have 4 limbs, who produce milk for their offspring, who have internal gestation with a placenta.

vs

Marsupial Mammals - animals that have bilateral symmetry, whose mouth forms second in early development, who have a notochord during development, who have backbones, who have jaws, who have lobe-limbs, who have 4 limbs, who produce milk for their offspring, who have external gestation in a pouch.

-----

Notice how all except the last part are exactly the same? Well, the placentals will have a common ancestor for their specific group, and the marsupials will have a common ancestor of their specific group. However, if we go back in time a little further, we will find a common ancestor of both groups (and if we throw in the monotremes, a common ancestor of all mammals), which would correspond with milk producing. If we cut off one clause at a time, we reach an even older common ancestor for the more general group: tetrapods, lobe-finned fish, jawed fish, all fish, chordates, all deuterostomes, and finally all bilateria (the common ancestor which would finally unite us with the insects and other arthropods, as well as molluscs and other groups).

So, while you may find such similarities as a one-way digestive system or having eyes more convincing, those similarities describe common ancestors for a broader group than the one focused on in this study: the placental mammals.

There was an ad' on tellie for coke or mars or something a few years ago where people all had tails. Set in Jamaica or some such place with Toots & the Maytals soundtrack I think. That was awesome, wish I could find that ad'.

There was an ad' on tellie for coke or mars or something a few years ago where people all had tails. Set in Jamaica or some such place with Toots & the Maytals soundtrack I think. That was awesome, wish I could find that ad'.

Haven't seen the ad, but the Toots track is most likely to be "Monkey Man". Toots and the Maytals did the original reggae. I liked Marley much, but Toots was better and still going strong, saw them on the telly performing at Glastonbury summer 2011. http://www.youtube.com/watch?v=7Wf0yKzpGAY

I saw the note in the article about Pangea which surprised me. I had the idea that Pangea was billions of years ago. So, I fired up Wikipedia (http://en.wikipedia.org/wiki/Pangaea) and found it was about 200 Million Years Ago. That seems a relativly short time to go from a single land mass to what we have today.

I saw the note in the article about Pangea which surprised me. I had the idea that Pangea was billions of years ago. So, I fired up Wikipedia (http://en.wikipedia.org/wiki/Pangaea) and found it was about 200 Million Years Ago. That seems a relativly short time to go from a single land mass to what we have today.

I really like this Internet thing.

And don't forget that Earth has gone from having a single land mass to multiple continents several times through history, or at least pre-history.

That ought to be exhibit one every time we hear "why can't we in the US have the same cool electronics as the Japanese" (most recently in the smartTV thread). Just cause it's available in Japan does not mean the world needs it...

Haven't seen the ad, but the Toots track is most likely to be "Monkey Man". Toots and the Maytals did the original reggae. I liked Marley much, but Toots was better and still going strong, saw them on the telly performing at Glastonbury summer 2011. http://www.youtube.com/watch?v=7Wf0yKzpGAY

Still off topic: actually I think the track was Broadway Jungle. When I saw Toots a year or two ago, he was still 'going' but I wouldn't say strong. Still, glad I got to see him.

Interactive Civilian wrote:So, while you may find such similarities as a one-way digestive system or having eyes more convincing, those similarities describe common ancestors for a broader group than the one focused on in this study: the placental mammals.

Thanks for your input and I agree with this common ancestors you have described for this thread. The thing is, we only have 4 billion years to have all these changes to happen from one common ancestor to another common ancestor. To me there's kinda not enough time to turned a rat form into a monkey form then from a monkey to a human form. When someone says it takes the whole 4 billion years to went from a monkey to a human, this, I would have convinced more. But please not that tiny little long tail rat? Cow got milk and a placental too. So we all came from the same common ancestors long long time ago? Hi, grandpa cow, your T-bone was good. We are eating our common ancestors. What a world. I'm not trying to be funny here, come to think of it, where is our morality these days? Sighs..

Do I still eat that T-bone? You bet you I will, not until the day for sure cow was our common ancestors, and until then.. :-)

Do I still eat that T-bone? You bet you I will, not until the day for sure cow was our common ancestors, and until then.. :-)

Ummm... no existing life form is your common ancestor. We share a common ancestor with cows, just as you and your cousin share a common ancestor (your grandparents). That common ancestor was not very cow-like, and in fact was probably quite similar to the artist's conception in the article.

So, it's not "hello, grandpa cow". It's more like "hello very distant cousin cow". If this bothers you, then I'm not really sure what to tell you. The facts don't really care much about how people feel about them, and they won't change because someone doesn't like the implications.

No offense intended, but it is rather clear from your posts that you don't have a very good understanding of evolution or biology. For one thing, you seem to think that one modern form evolves into another (something like rat to cow to monkey to human) from some kind of "lower" life form into a "higher" life form (quotes because such ideas are meaningless in modern biology). This is not the case at all. Every single existing life form today, from the smallest bacterium to the largest blue whale and also including us is equally evolved. They are all the tips of the branches of a very large tree. We all began (and no, we don't know exactly how, yet) ~3.8 billion years ago as a single-celled bacteria-like organism and have all traveled our own branching paths to the present forms that exist today. If you don't think that is enough time, then it might be worth it to spend some time brushing up on your basic biology to build up your fundamental understanding of how organisms function and then focusing on genetics for a bit to get a clearer picture of the fundamental basis of evolutionary change and where it comes from. Then, when you have that fundamental basis to work from, focus on how evolution works.

You'll not only find that 3.8 billion years is more than enough time to explain the diversity we see today, but you'll probably be left wondering why there isn't so much more (the answer to that basically being, things go extinct).

Like I said, I mean no offense. However, it is difficult to engage these kinds of topics critically if you don't have the correct understanding of the fundamentals. I say this because it sounds like the things you are saying you are not convinced of are really things that no one (except usually creationists building strawmen) is saying are happening or have happened.

Have a nice day, and eat what you are comfortable with. It doesn't matter what, as they are all (animals, fungi, plants, the bacteria in your yogurt) distant cousins to one degree or another.

I saw the note in the article about Pangea which surprised me. I had the idea that Pangea was billions of years ago. So, I fired up Wikipedia (http://en.wikipedia.org/wiki/Pangaea) and found it was about 200 Million Years Ago. That seems a relativly short time to go from a single land mass to what we have today.

I saw the note in the article about Pangea which surprised me. I had the idea that Pangea was billions of years ago. So, I fired up Wikipedia (http://en.wikipedia.org/wiki/Pangaea) and found it was about 200 Million Years Ago. That seems a relativly short time to go from a single land mass to what we have today.

I really like this Internet thing.

If you have an iPad, there's are really nifty (and FREE) app called EarthViewer which lets you move through time over the past 4.5 billion years years to see the layout of the continents, as well as atmospheric Oxygen and Carbon Dioxide levels. It's VERY cool if you want better visuals on how the earth has changed over time.

The thing is, we only have 4 billion years to have all these changes to happen from one common ancestor to another common ancestor.

Remember that it's not a serial process. It doesn't change from Thing A to Thing B to Thing C in a chain. Rather, it's highly parallel.... trillions of individual creatures, with billions of tiny adaptations, pulling their species in the direction of those which are most successful.

If you look at timescales, like the one helmingstay mentions, one of the things that really sticks out is that evolving the initial single-celled organisms took a really, really long time, and then evolving to multicellular life took a huge long time again, and then as soon as multicellular life was functioning, there was an absolute explosion in diversity. Things just get faster and faster and faster.

Developing the basics from nothing was extremely difficult, but adding things like self-reflective intelligence in humans was really, really easy. Getting cells working was, at least from what we can see, an extraordinarily unlikely chain of events, judged purely from how long it took for random processes to hit on a working approach. Intelligence and civilization seem to be trivial in comparison.

We, as the civilized intelligences, like to think that's the most important part, but it's the basic cellular machinery that's the really impressive part of the whole deal.

Well, gee, JTM, that happened a really long time ago, and being definitive about why the dinosaurs are dead is pretty darn difficult.

It's like trying to solve a giant murder mystery that happened 65 million years ago. That's long enough for the continents themselves to be much, much different than they were. If actual entire continents can be torn apart in that kind of timeframe, imagine what that does to regular evidence.

It's not like they can play a boardgame for an hour and announce that it was Miss Scarlet in the library with a candlestick.